1. Field of the Invention
The present invention relates to a device for reducing a concentration of carbon monoxide contained in reformate gas that has been obtained through reformation of a hydrocarbon such as methyl alcohol and a fuel such as water vapor. The present invention also related to a carbon monoxide reducing method.
2. Description of Related Art
As a system using this type of reformate gas, there is known a system that employs a fuel cell. For example, a typical fuel cell employing an electrolyte film permeable to protons is designed to electrochemically oxidize fuel, such as hydrogen gas, and to take out an electromotive power generated during the process.
A reformer that is used in such a system equipped with this fuel cell is designed, for example, to supply a reforming catalyst with methanol and water vapor and to obtain a reformate gas therefrom composed mainly of hydrogen, through a reforming reaction under a predetermined temperature. Basically, the reforming reaction is an endothermic reaction in which 1 mole of methanol reacts with 1 mole of water to produce 3 moles of hydrogen and 1 mole of carbon dioxide. However, a small amount of carbon monoxide is inevitably generated. The reaction is expressed by the formula (i) shown below. EQU CH.sub.3 OH+H.sub.2 O.fwdarw.3H.sub.2 +CO.sub.2 +.alpha.CO-49.5(kJ/mol) (i)
In the case where reformate gas is used as a fuel gas for a fuel cell, platinum is employed as a catalyst for electrodes of the fuel cell. Therefore, if the fuel gas or reformate gas contains carbon monoxide, the carbon monoxide poisons and deteriorates the platinum. For this reason, it is preferable to gradually remove carbon monoxide from the reformate gas. Various methods for removing carbon monoxide from the reformate gas are known in the art. For example, Japanese Patent Application Laid-Open No. HEI 8-329969 discloses a device wherein an oxidation portion for oxidizing carbon monoxide contained in the reformate gas is disposed between a reforming portion for causing a reforming reaction and a fuel cell. In this device, the concentration of carbon monoxide is detected in a reformate gas feed path extending from the reformer to the fuel cell. If the concentration has become equal to or greater than a predetermined level, the amount of air supplied to the oxidation portion is increased.
In the aforementioned device of the related art, the carbon monoxide contained in the reformate gas reacts with oxygen contained in the air at an oxidation catalyst of the oxidation portion and is converted to carbon dioxide, which is not harmful to the platinum. In this manner, carbon monoxide is removed from the reformate gas. However, according to the aforementioned device of the related art, although the amount of air blown into the oxidation portion is increased with increases in concentration of carbon monoxide contained in the reformate gas, that amount of air may not always be suited for an actual amount of carbon monoxide.
That is, in principle, 1 mole of carbon monoxide reacts with 1/2 mole of oxygen to produce 1 mole of carbon dioxide. Therefore, if the amount of air supplied to the oxidation portion is insufficient with respect to an amount of carbon monoxide contained in the reformate gas, not all of the carbon monoxide is converted to carbon dioxide, and unreacted carbon monoxide is supplied to the fuel cell and poisons the platinum catalyst. Conversely, if an excessive amount of air is supplied to the oxidation portion, surplus oxygen is supplied to the fuel cell together with the reformate gas. As a result, the fuel gas for the fuel cell may burn instead of being used to generate electric power. However, in the device of the related art wherein the amount of air blown into the oxidation portion is increased simply based on a detected concentration of carbon monoxide, no effective measures may be taken to address or eliminate these problems.